1. Field of the Invention
The present invention relates generally to detection of interference due to intermodulation products of transmitted signals in a wireless system, and more specifically, but not exclusively, to detection of Passive Intermodulation (PIM).
2. Description of the Related Technology
Intermodulation products may be generated in a wireless system when two or more signals are transmitted at different frequencies along a signal path including a component having a non-linear transmission characteristic; these products differ in frequency from the signals from which they were generated, and may potentially cause interference to other signals. The generation of intermodulation products is becoming a problem of increasing importance in modern wireless communication systems, and in particular cellular wireless systems, since the radio frequency spectrum available has been steadily expanded as additional bands have become available, and the pattern of allocation of uplink and downlink bands within the available spectrum for use by various cellular systems, such systems using GERAN (GSM EDGE Radio Access Network), UTRAN (UMTS Terrestrial Radio Access Network) and E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) radio access networks, and by various operators, is complex and territorially dependent. In this environment, and in particular at a cellular radio base station, it is likely that circumstances arise in which intermodulation products generated from transmitted carriers in one or more downlink bands would fall within an uplink band in which signals are received at the base station. Intermodulation generated by non-linear characteristics of active components such as power amplifiers may generally be dealt with at a design stage by appropriate frequency management and filtering, but intermodulation caused by non-linear characteristics of passive components, so called passive intermodulation (PIM), may prove more difficult to manage. Many passive components may exhibit a non-linear transmission characteristic to some degree, for example due to an oxide layer at a metal to metal contact, and the non-linear characteristic may develop with time as an ageing process of the component. Furthermore, PIM may be caused by components in the signal path between the transmitter and the receiver that are external to the transceiver equipment and which may be outside the operator's control, such as metallic objects on the antenna tower or other objects in the signal path through the propagation environment, such as fences; this is known as the “rusty bolt” effect.
Interference due to PIM may reduce the carrier to interference ratio at a receiver, which may reduce coverage by a cell significantly. As a solution to the problem of suspected interference caused by PIM in a cellular radio network, the source of the PIM may be tracked down by field engineers and a component causing the PIM may be replaced. However, it is typically difficult to distinguish the effects of interference due to PIM from other types of interference, such as interference from neighbouring cells, and from other types of degradation of receiver performance, such as a raised noise floor. Detecting and tracking down PIM is particularly difficult if the PIM is generated in the propagation environment, especially if the interference due to PIM is an intermittent fault, for example being dependent upon weather conditions.
PIM may be conventionally detected and tracked down using test equipment which generates PIM by transmitting high power sinewave test signals and which uses a sensitive receiver to detect the presence of PIM by tuning to frequencies at which intermodulation products of the signals may be expected, and detecting an increase in received power when the test signals are transmitted. However, such systems are intrusive, in that they involve the transmission of test signals which may disrupt the normal operation of the wireless network, and they typically require that a received channel of interest, in which intermodulation products are to be detected, should be an unoccupied channel.
Aspects of the invention address at least some of the limitations of the prior art detection systems.